The growing adoption of Power BI Desktop is gathering new users of the DAX language. At the same time, there are a few new features, such as bidirectional filter propagation, that introduce new concepts to existing knowledge. For this reason, in the last weeks we published two articles describing important basic concepts and clarifying the behavior of filter context propagation with the bidirectional filter.
A more complete description is included in our new book, The Definitive Guide to DAX, which will be available in October 2015.